During its development a plant shoot progresses through a juvenile phase and a adult phase and then begins to produce reproductive structures. The vegetative phases of development are marked by distinctive differences in the morphology and physiology of leaves produces during each phase, and by a differences in the reproductive competence of the shoot apical meristem. We are studying the regulation of this process in Arabidopsis by identifying and characterizing mutations that affect a phase-specific pattern of trichome distribution. Genes identified in this screen may act either to regulated the identity of a leaf, or more generally, to regulate the transition form a juvenile to adult phase of shoot development. We propose to complete a phenotypic and genetic characterization of several "early trichrome" mutations in order to determine whether they regulated leaf identity or phase change, and how they interact to determine phase- specific patterns of vegetative development. To begin to dissect the molecular mechanism of phase change, we will characterize the function of two genes, HASTY and EARLY TRICHOMES. Specifically, we will examine where they are expressed in the plant, whether they function cell-autonomously or non-cell-autonomously to regulate the developmental identity of the shoot, and how their expression is affected by various genetic, hormonal and environmental factors that may play a role in phase change. Transgenic lines containing the GL4 transcriptional activator under the control of a constitutive promoter or tissue-and organ-specific regulatory sequences will be generated and used to examine the effect of expressing HASTY and EARLY TRICHOMES in spatially restricted domains of the plant. In addition, we will generate transgenic plants that contain a readily observable marker, such as green fluorescent protein, under the control of EARLY TRICHOMES regulatory sequences. These plants will be used to characterize the effect of existing mutations on the expression of this gene, and to isolate new mutations in genes upstream of EARLY TRICHOMES. The temporal regulation of developmental transitions is poorly understood in most organisms. The experiments described in this proposal will provide the first evidence of the molecular nature of the developmental clock that controls vegetative phase changes in Arabidopsis, and will lay the foundation for future studies of this fundamental problem in developmental biology.